This invention relates to ultrasonic transducers for nondestructive evaluation and their fabrication, and especially to dark field transducers to inspect a solid part for flaws and cracks.
Several NDE techniques such as eddy current, fluorescent penetrant, magnetic particle, and ultrasonic surface wave, exist for locating near surface cracks in parts. With some of these it is possible to measure crack length and to measure average crack depth. It is desirable, in addition to determining crack shape, to measure crack depth along the crack in order to make cost effective part rejection decisions and to provide information for fatigue predictions. Existing methods of NDE do not provide capability for measuring crack depth profiles, and there is a need to estimate the size of other types of surface flaws.
In the field of optics, dark field illumination is known in which the illuminating beam is a hollow cone of light; the object is viewed with a center lens which collects scattered light. The principle has not been applied to ultrasonic transducers, however.